This invention relates to a method for the separation of magnesium hydroxide from a mixture of particulate magnesium hydroxide and dissolved sodium sulphate in water and for the washing and thickening of the magnesium hydroxide.
Magnesium hydroxide (also known as milk of magnesia) is a product of the reaction of magnesium sulphate and sodium hydroxide as follows: EQU MgSO.sub.4 +NaOH.fwdarw.Mg(OH).sub.2 +Na.sub.2 SO.sub.4
Once the reaction is complete, the soluble sodium sulphate must be separated from the particulate magnesium hydroxide.
In the existing precipitation process, this separation is achieved by conventional filtration techniques where the particles of magnesium hydroxide are retained by a filter surface while the sodium sulphate passes through and constitutes the wasted filtrate. Water is added in batch mode or continuously, thereby decreasing the sulphate concentration in the magnesium hydroxide to less than 0.1% (m/v). This process takes typically 3 to 4 days to prepare a batch of 2700 liters of Mg(OH).sub.2 suspension with a concentration of ca. 8% m/m and consumes great quantities of water and steam. Once the required concentration of sodium sulphate is achieved, water addition is stopped and the filtration process is allowed to continue, thereby progressively increasing the concentration and viscosity of the magnesium hydroxide. When the viscosity reaches the desired level, the magnesium hydroxide is ready for final concentration adjustment prior to packing.
There is a need for a new method for the recovery, of magnesium hydroxide from the reaction mixture.